Prepubertal levels of gonadotropins were previously unmeasurable due to the sensitivity of available assays. Immunochemiluminimetric assay (ICMA) is able to measure LH levels as low as 0.01 IU/L and has minimal cross reactivity with FSH, TSH or the alpha subunit. Utilizing ICMA we will measure LH secretion during two 6 h periods, one from 0800-1400 and the other 2000-0200 h. We postulate that the GnRH pulse generator is active at an adult frequency, at a reduced amplitude, in healthy prepubertal children, and the dirnal rhythm is present. These are precisely our findings. Preliminary data show marked changes in body composition and the regional distribution of body fat as well as alterations in GH and leptin levels.